DyeSensitized Tandem Solar Cells with Extremely High OpenCircuit ...

Full Paper DOI: 10.1002/ijch.201400204

Dye-Sensitized Tandem Solar Cells with Extremely High Open-Circuit Voltage Using Co(II)/Co(III) Electrolyte Won Seok Choi†,[a] In Taek Choi†,[a] Ban Seok You,[b] Ji-Woon Yang,[b] Myung Jong Ju,*[a] and Hwan Kyu Kim*[a] Abstract: For achieving a high open-circuit voltage (Voc) in dye-sensitized tandem solar cells, series-connected tandem solar cells were fabricated. In order to optimize series-connected tandem solar cell systems, the current density of the top and bottom cells should be well matched to be identical, and the Voc of each of the cells should also be as high as possible. Furthermore, the top cell should be transparent and the bottom cell should have the longer-wavelength absorption, for utilizing only the light passing through the top

cell. This leads to a high Voc. In this study, we report dyesensitized tandem solar cells having an extremely high Voc using the Co(bpy)32 + /3 + (bpy = 2,2’-bipyridine) redox couple. Dye-sensitized tandem solar cells employing JK303/HC-A1 with the Co(bpy)32 + /3 + redox couple as the top cell and N749/HC-A4 with the I¢/I3¢ redox couple as the bottom cell were shown to have an extremely high Voc of > 1.66 V, the highest value for dye-sensitized tandem solar cells reported to date.

Keywords: dye-sensitized solar cells · redox couples · renewable resources · semiconductors · sensitizers

1. Introduction During the 21st century, the problem of energy will become much more serious because of depletion of the worldÏs oil resources and the rapidly increasing energy demand. For this reason, developing clean and renewable energy sources has become one of the greatest challenges for our modern society. Technologies that harvest the clean, renewable and abundant solar power are among the most attractive methods to solve this energy problem. Among them, solar cells, which directly convert solar energy to electricity, have emerged as a promising candidate. In the past two decades since the first prototype was reported by M. Grtzel in 1991,[1]considerable efforts have been devoted to the dye-sensitized solar cell (DSSC). DSSCs have received considerable attention as a new generation of sustainable photovoltaic devices because of their high incident solar power conversion efficiency (PCE), colorful and decorative nature, and low cost of production.[2–6]However, DSSC efficiency is far behind that of crystal or microcrystal silicon solar cells. One of the methods to improve the efficiency is to introduce a tandem or a hybrid structure into DSSCs, where a front electrode absorbs short-wavelength light and a back-side electrode absorbs long-wavelength light.[7,8] To date, there have been many reports to improve the efficiency of dye-sensitized tandem solar cells, such as double-sided FTO,[8] double-layer structure,[9–12] p-n structure,[13] and hybrid tandem solar cells like dye-sensitized structures combined with Cu(InxGa1¢x)Se2 (CIGS)[14] or n-GaAs/AlxGa(1¢x)As (GGC).[15] The PCE values of these Isr. J. Chem. 2015, 55, 1002 – 1010

tandem solar cells are relatively high, but their fabrication processes are too complicated and expensive. For this reason, the expansion of the absorption region of the sensitizing dye was mainly studied as the conventional method for increasing the photocurrent density (Jsc). By employing this concept, some panchromatic absorption complexes were reported as sensitizers in DSSCs.[16–19] However, these dyes could not give sufficiently high open-circuit voltage (Voc). To improve the Voc, here we investigate series-connected tandem DSSCs (Figure 1). In this system, the top cell is made up of a transparent cell and the bottom cell should have a longer-wavelength absorption compared to the top cell, for utilizing only the light passing through the top cell. Generally, in the parallel connection, the total current density is the sum of the current densities of the top and bottom cells, but the Voc is supplied by the low-voltage [a] W. S. Choi, I. T. Choi, M. J. Ju, H. K. Kim Global GET-Future Laboratory Department of Advanced Materials Chemistry Korea University Sejong 339-700 (Republic of Korea) e-mail: [email protected] [email protected] [b] B. S. You, J.-W. Yang Department of Electronics and Information Engineering Korea University Sejong 339-700 (Republic of Korea) [†] These authors contributed equally to this work.

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Full Paper were purchased from Sigma Aldrich Inc. (USA) and used as received without further purification. 2.2. DSSC Fabrication

Figure 1. Schematic representation of the series-connected tandem DSSC structure consisting of two different DSSCs.

cell. With regard to the photon energy, a shorter-wavelength photon has higher energy compared to a longerwavelength photon. However, parallel-connected tandem DSSCs cannot utilize this characteristic. In order to obtain a high Voc, series-connected tandem cells are necessary. In the series-connected tandem DSSCs, the Voc becomes the sum of the Voc of the top and bottom cells, but the current densities of the top and bottom cells should be well matched and ideally identical, because the photocurrent of the tandem cell is determined by the smaller of the two photocurrents. Furthermore, the top cell is required to have a high voltage and the bottom cell should have a longer-wavelength absorption compared to the top cell. If we can satisfy these requirements, we can obtain DSSCs with a high Voc. In this study, we report series-connected tandem DSSCs with an extremely high Voc of > 1.66 V, the highest value for dye-sensitized tandem solar cells reported to date.

FTO plates (Pilkington, fluorine tin oxide glass, 8 W cm¢2) were cleaned in detergent solution, water, and ethanol using an ultrasonic bath. The FTO substrates were immersed in 40 mM aqueous TiCl4 solution at 708 for 30 min and washed with water and ethanol. A TiO2 colloidal paste (Solaronix, Ti-Nanoxide T/SP, 20 nm particle size) was screen printed onto FTO/glass and sintered at 5008 for 30 min in air. The thickness of the transparent layer was measured using an Alpha-Step 250 surface profilometer (Tencor Instruments, San Jose, CA), and a paste for the scattering layer containing 500 nm sized anatase particles (ENB Korea, STP-500N) was deposited by doctor-blade printing and then dried for 2 h at room temperature. The TiO2 electrodes were sintered at 5008 for 30 min. The films of the top cells were composed of a transparent layer. These were immersed in a solution of 3 × 10¢4 M N719 dye, 2 × 10¢4 M NKX2677 dye containing 7 × 10¢3 M HC-A3, or 3 × 10¢4 M JK303 dye containing 3 × 10¢4 M HC-A1 in a mixture of ethanol and THF (volume ratio, 2 : 1) at room temperature for 12 h. The films of the bottom cells were immersed in 2 × 10¢4 M ethanolic N749 solution containing 1 × 10¢3 M HC-A4 at room temperature for 20 h. The dye-adsorbed TiO2 photoanodes were assembled with Pt using a thermal adhesive film (25 mm thick Surlyn, DuPont) as a spacer to produce a sandwichtype cell. Electrolyte solution was introduced through a drilled hole on the counter electrodes (CEs) via vacuum backfilling. The hole was sealed with cover glass using Surlyn. For the maximum open-circuit voltage (Voc) of the tandem DSSC, Co(II)/Co(III) and I¢/I3¢ redox couples were used in the top and bottom DSSCs, respectively. 2.3. Photovoltaic Measurements

2. Experimental Section 2.1. Materials

The sensitized dyes N719 [RuL2(NCS)2, L = 2,2’-bipyridine-4,4’-dicarboxylate] and N749 [RuL’(NCS)3, L’ = 2,2’ : 6’,2’’-terpyridine-4,4’,4’’-tricarboxylate] were purchased from Ohyoung Industrial Co., Ltd. (Seoul, Korea) and Solaronix S. A. (Lausanne, Switzerland), respectively. The organic sensitizers NKX2677[20] and JK303,[21] coadsorbents HC-A1,[22] HC-A3,[23] and HC-A4,[23] and Co(bpy)32 + /3 + (bpy = 2,2’-bipyridine) complexes[24] were prepared by reported procedures. Tetrahydrofuran (THF) was dried over and distilled from sodium and benzophenone under an atmosphere of dry nitrogen. Other reagents and chemicals including 1,2-dimethyl-3-propylimidazolium iodide (DMPII), LiI, I2, and 4-tert-butylpyridine Isr. J. Chem. 2015, 55, 1002 – 1010

Photoelectrochemical data were measured using a 1000 W xenon light source (Oriel, 91193) that was focused to give 100 mW cm¢2, which is the equivalent of one sun at air mass (AM) 1.5 G at the surface of the test cell. The light intensity was adjusted with a Si solar cell that was double-checked with an NREL-calibrated Si solar cell (PV Measurement Inc.). The applied potential and measured cell currents were measured using a Keithley model 2400 digital source meter. The current¢voltage (J¢V) characteristics of the cell under these conditions were determined by biasing the cell externally and measuring the generated photocurrent. This process was fully automated using the Wavemetrics software. The measurement-settling time between applying a voltage and measuring a current for the J¢V characterization of DSSCs was fixed at 50 ms. A similar data acquisition system was


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Full Paper used to control the incident photon-to-current conversion efficiency (IPCE) measurement. Under full computer control, light from a 300 W Xe lamp was focused through a high-throughput monochromator onto the photovoltaic cell under test. The monochromator was incremented through the visible spectrum to generate the IPCE (l) curve as expressed in Eq. (1), IPCE ðlÞ ¼ 1240 ðI sc =lf Þ

ð1Þ

where l is the wavelength, Isc is the current at short circuit (mA cm¢2), and f is the intensity of the monochromatic light (W m¢2). The IPCE curve can be derived from the measured absorption spectrum of the DSSC for comparison.

3. Results and Discussion 3.1. Properties of Sensitizers and Coadsorbents

Figure 2 shows the molecular structures and UV¢vis absorption spectra of various sensitizers and coadsorbents used in this study. An effective sensitizer for a DSSC must be able to absorb a significant portion of the solar spectrum and inject excited electrons into a semiconducting scaffold faster than they relax to the sensitizerÏs ground state. Among the metal complexes, Ru complexes[25–27] have shown the best photovoltaic properties: broad absorption spectra, suitable excited- and groundstate energy levels, relatively long excited-state lifetimes, and good electrochemical stabilities. To further improve the efficiency of DSSCs, enlarging the spectral absorption

Figure 2. (a,b) Molecular structures of sensitizers and coadsorbents used in this study. (c,d) Corresponding UV¢vis absorption spectra. Isr. J. Chem. 2015, 55, 1002 – 1010



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Full Paper region of the sensitizer to the near-IR region is desirable. In 1997, Grtzel and co-workers developed the N749 dye,[28] also called the “black dye”, in which the Ru center has three thiocyanato ligands and one terpyridine ligand substituted with three carboxyl groups. Based on this dye, an IPCE spectrum was obtained over the whole visible range extending into the near-IR region (Figure 2c). Nazeeruddin et al.[29] demonstrated that the doubly protonated form, (Bu4N)2[Ru(dcbpyH)2(NCS)2], named N719, exhibited an increased PCE compared to N3 dye. It achieves near-unity charge transfer into TiO2 due to chemisorption of the dyeÏs carboxylate groups onto the TiO2 surface, facilitating electron injection.[30–33] As an alternative to Ru complexes, organic dyes exhibit many advantages as sensitizers: (i) the molecular structures of organic dyes are diverse and can be easily designed and synthesized, (ii) organic dyes are superior to noble metal complexes in terms of cost and environmental issues, and (iii) they have large absorption coefficients due to intramolecular p¢p* transitions. Generally, a donor¢p-bridge¢acceptor (D¢p¢A) structure is the common characteristic of these organic dyes. With this construction it is easy to design new dye structures, extend the absorption spectra, adjust the HOMO and LUMO levels and complete the intramolecular charge separation. When a dye absorbs light, intramolecular charge transfer occurs from subunit D to A through the p bridge. For n-type DSSCs, the excited dye injects the electron into the conduction band of the semiconductor via the electron-acceptor group, A. However, in p-type DSSCs, the excited dye captures the electron from the valence band of the semiconductor to complete the interfacial charge transfer. Many efforts have been made to change the different parts of organic dyes to optimize DSSC performance. Hara and co-workers developed an organic sensitizer (NKX2677) consisting of a coumarin unit and a cyanoacrylic acid unit linked by thienyl units.[34,35] The coumarin unit acts as electron donor. The cyanoacrylic acid group is considered as the electron acceptor due to the strong electron-withdrawing ability of cyano and carboxyl groups. In addition, the lengthening of the sensitizer with the thienyl units enhances aggregation between sensitizers on the TiO2 surface, resulting in a decrease of electron injection yield owing to the intermolecular charge transfer. Thus, aggregation of the coumarin sensitizer should be inhibited by the use of a coadsorbent.[17,35] The number of thiophene units did not influence the absorption spectra of NKX2677; however, it greatly affected the photovoltaic properties owing to the issue of different aggregation behavior on the TiO2 surface. Absorption peaks (lmax) were observed at 511 nm, and the molar absorption coefficient at lmax was 64,300 M¢1 cm¢1 for NKX2677.[20] One of the weaknesses of organic sensitizers is the sharp and narrow absorption bands in the blue region of Isr. J. Chem. 2015, 55, 1002 – 1010

their UV spectra, which decreases their light-absorption capabilities. Therefore, molecular engineering of organic sensitizers is required in order to broaden and redshift their absorption spectra for high conversion efficiency.[21,36–38] To achieve this, K. Lim et al. developed an organic sensitizer (JK303) consisting of 2-hexyloxy-6,8,8-trimethyl-3,8-dihydrocyclopenta[a]indene as an electron donor and cyanoacrylic acid as an electron acceptor, linked by a planar aryl linker.[21] JK303 exhibited an increased spectral response in the red region of the solar spectrum (Figure 2c) due to the planar p-conjugated unit in the bridged framework,[39,40] and it can also use the Co(II)/Co(III) redox couple because of the long alkyl chains.[24] In DSSCs, the combination of dye molecules with coadsorbents gives a more compact monolayer than the dye layer alone, thus blocking the vacant sites on the TiO2 surface. Coadsorbents also prevent aggregation phenomena of the dye molecules and, in this manner, they promote a more favorable packing by occupying the empty space between dye molecules. Moreover, the use of coadsorbents suppresses the surface protonation of the dyecoated TiO2 film and, as a result, the degradation of the chemisorbed dye molecules. Consequently, the use of efficient coadsorbents is a powerful tool towards the improvement of the overall photovoltaic performance and long-term stability of DSSCs. In 2011, B. J. Song et al. published a work on the synthesis and evaluation of a carbazole-based coadsorbent,[41] 4-{3,6-bis[4-(2-ethylhexyloxy)phenyl]-9H-carbazol-9-yl}benzoic acid (HCA1), which has a bulky Y-shaped structure and strongly absorbs UV light. As a result, HC-A1 can efficiently harvest light and act as a sensitizer in the short-wavelength regions. Equally important, when used in DSSCs in combination with other purely organic photosensitizers, it was shown that it can cover the empty coordination sites on TiO2, thus reducing charge recombination and, furthermore, suppressing the p¢p stacking of the other organic photosensitizer in the pores of TiO2 films. In comparison with deoxycholic acid (DCA), HC-A1 was found to improve both the Voc and Jsc values more significantly in DSSC devices constructed under similar experimental conditions. In 2013, the same research group published two analogous triarylamine-based compounds with p-conjugated phenylene or naphthalene aryl linkers.[23] Both HC-A3 and HC-A4 were found to suppress dye aggregation and reduce charge recombination, albeit less effectively compared to HC-A1.[22,23,42] These coadsorbents were also able to prevent the p¢p stacking of the simultaneously used dye molecules and reduce charge recombination upon cosensitization. Moreover, they were shown to have a higher light-harvesting effect at wavelengths of 325–400 nm (Figure 2d) than HC-A1, leading to an increase in the corresponding Jsc values.



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Full Paper 3.2. Photovoltaic Properties of the Single DSSCs

The performance of the single DSSCs with various sensitizers and coadsorbents was measured. Figure 3 shows the J¢V characteristics and corresponding IPCE curves of the DSSCs with different sensitizers and coadsorbents, and the photovoltaic performance data are summarized in Table 1. Except for the JK303/HC-A1-based DSSC, all DSSCs were fabricated with the I¢/I3¢ redox couple. The N719-based DSSC exhibited a Jsc of 17.2 mA cm¢2, a Voc of 740 mV, an FF of 72.7 %, and a PCE of 9.3 %. The NKX2677/HC-A3-based DSSC showed a Jsc of 15.4 mA cm¢2, a Voc of 702 mV, an FF of 78.5 %, and a PCE of 8.6 %, with a current value that was apparently lower than that of the N719-based DSSC, as can be seen in the IPCE curve (Figure 3b), and the Voc was also much lower than that of the N719-based DSSC. The JK303/HCA1-based DSSC with Co(bpy)32 + /3 + (bpy = 2,2’-bipyridine) redox couple exhibited a relatively lower current compared to the previous two DSSCs assembled with the I¢/I3¢ redox couple, whereas its Voc exhibited the highest value, which was ascribed to the higher redox potential of Co(bpy)32 + /3 + than the I¢/I3¢ redox couple (inset in Figure 3a). The IPCE value for the JK303/HC-A1-based DSSC at the shorter-wavelength range of about 380 nm is higher than those of the other two DSSCs, which can be

favorable for absorption in the UV¢vis region, when it is used as the top cell in tandem DSSCs. The N749-based DSSC showed a Jsc of 16.3 mA cm¢2, a Voc of 676 mV, an FF of 67.8 %, and a PCE of 7.5 %. To reduce the number of possible charge-recombination pathways occurring at the TiO2/dye/electrolyte interface, here we introduced DCA as a coadsorbent. DCA is often used as a coadsorbent to break up dye aggregation, thereby significantly improving both Voc and Jsc.[34,43–45] As can be seen, the N749/DCA-based DSSC showed higher Jsc and Voc values than the N749-based DSSC (Figures 3c and 3d, Table 1). As shown in Figure 3d, at wavelengths longer than 800 nm, the DSSCs with N749 produced higher IPCE values than the DSSCs with the other three sensitizers (Figure 3b). Moreover, the Jsc and Voc of the N749/HC-A4-based DSSC significantly increased, when HC-A4 was used as coadsorbent instead of DCA. This implies that HC-A4 can act as a more effective spacer than DCA among dye molecules and thus suppresses the p¢p stacking of the dye molecules, which in turn retards the charge recombination and hence improves the device performance, such as Jsc and Voc, significantly (Figures 3c and 3d). The photovoltaic performance of DSSCs was measured with a black metal mask (0.16 cm2). [a] Electrolyte: 0.6 M DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyri-

Figure 3. (a) J¢V characteristics of the DSSCs with various dyes and coadsorbents: N719 (black); JK303/HC-A1 (red); NKX2677/HC-A3 (blue). The inset shows the redox potential levels of the I¢/I3¢ and Co(bpy)32 + /3 + redox couples. (b) Corresponding IPCE action spectra. (c) J¢V characteristics of the N749-based DSSCs with different coadsorbents. (d) Corresponding IPCE action spectra. Isr. J. Chem. 2015, 55, 1002 – 1010



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Full Paper Table 1. Photovoltaic parameters of the single DSSCs with sensitizer only and with various coadsorbents under one-sun illumination. Device

Jsc (mA cm¢2)

Voc (mV)

FF (%)

PCE (%)

N719[a] NKX2677/HC-A3[a] JK303/HC-A1[b] N749[a] N749/DCA[a] N749/HC-A4[a]

17.2 15.4 13.5 16.3 16.9 17.4

740 702 897 676 747 752

72.7 78.5 72.4 67.8 71.6 71.3

9.3 8.6 8.7 7.5 9.0 9.3

dine (TBP) in CH3CN. TiO2 thickness: 16 mm (active layer 12 mm and scattering layer 4 mm). [b] Electrolyte: 0.22 M Co(bpy)32 + , 0.05 M Co(bpy)33 + , 0.05 M LiClO4, and 0.8 M TBP in CH3CN. TiO2 thickness: 7 mm (active layer 5 mm and scattering layer 2 mm). 3.3. Photovoltaic Properties of Tandem DSSCs

In view of the above results, we assembled series-connected tandem (ST) DSSCs. Figure 4 shows the J¢V characteristics of the tandem DSSCs and corresponding IPCE action spectra. In general, the Voc of a series-connected tandem DSSC is determined by adding the Voc of the top cell to that of the bottom cell. Since the FF of a seriesconnected tandem DSSC is determined by current matching of the Jsc values of the top and bottom cells, the Jsc of

the top cell should be equal to that of the bottom cell. Thus, for a high-PCE tandem DSSC, the Jsc values of the top and bottom cells should simultaneously be identical and high. In order to ensure good matching between the top and bottom cells, the photocurrent of each cell in the tandem DSSC structure was controlled by changing the thickness of the light-absorbing nanocrystalline TiO2 electrode, resulting in good photocurrent matching and high efficiency (Table 2). The tandem DSSC with N719 (top) and N749/HC-A4 (bottom) showed a PCE of 8.9 % with a Jsc of 8.4 mA cm¢2, a Voc of 1,469 mV and an FF of 71.8 % under 100 mW cm¢2 simulated sunlight. The tandem DSSC with NKX2677/HC-A3 (top) and N749/ HC-A4 (bottom) showed a PCE of 8.8 % with a Jsc of 8.4 mA cm¢2, a Voc of 1,495 mV and an FF of 70.2 % under the same conditions. The PCE values of the two

Figure 4. (a) J¢V characteristics of the tandem DSSC with N719 (top) and N749/HC-A4 (bottom). (b) Corresponding IPCE action spectra. (c) J¢V characteristics of the tandem DSSC with NKX2677/HC-A3 (top) and N749/HC-A4 (bottom). (d) Corresponding IPCE action spectra. Isr. J. Chem. 2015, 55, 1002 – 1010



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Full Paper Table 2. Photovoltaic parameters of the individual DSSCs and the corresponding tandem DSSCs under one-sun illumination. Device

Jsc (mA cm¢2)

Voc (mV)

FF (%)

PCE (%)

Top cell (N719)[a] Bottom cell (N749/HC-A4)[b] Tandem cell Top cell (NKX2677/HC-A3)[a] Bottom cell (N749/HC-A4)[b] Tandem cell

8.3 8.7 8.4 9.2 8.1 8.4

764 704 1,469 762 720 1,495

71.8 73.9 71.8 66.3 75.6 70.2

4.6 4.5 8.9 4.7 4.4 8.8

tandem cells are similar, whereas the FF and Voc values are quite different. The difference in the Jsc values of the top and bottom cells is relatively small for the tandem DSSC with N719 (top) and N749/HC-A4 (bottom), so the FF is higher than that of the tandem DSSC with NKX2677/HC-A3 (top) and N749/HC-A4 (bottom) because the FF is dependent on the current matching between top and bottom cells. On the other hand, the Voc of the latter tandem cell is higher than that of the former. The photovoltaic performance of DSSCs was measured with a black metal mask (0.16 cm2). [a,b] Electrolyte: 0.6 M DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in CH3CN. [a] TiO2 thickness: 2.5 mm (active layer). [b] TiO2 thickness: 16 mm (active layer 12 mm and scattering layer 4 mm). For further increasing the Voc, we prepared a seriesconnected tandem DSSC in which the top cell was made with JK303/HC-A1 using the Co(bpy)32 + /3 + redox couple and the bottom cell with N749/HC-A4 using the I¢/I3¢ redox couple, which can absorb the light that has passed through the top cell (Figure 5a). The J¢V characteristics and corresponding IPCE curves are shown in Figures 5b and 5c, respectively. Here, the TiO2 film thickness of the top cell was adjusted to provide as similar photocurrent values for the top and bottom cells as possible. The top cell, with JK303/HC-A1, exhibits a spectral response in the visible-wavelength region, whereas the bottom cell, with N749/HC-A4, shows an efficient response in the near-infrared region (Figure 5c). As a result, when the two cells were connected in series to produce a tandem cell, a Jsc of 10.3 mA cm¢2, a Voc of 1,664 mV and an FF of 63.1 % were obtained from the J¢V curve (Table 3), leading to the highest overall PCE of 10.8 % among the tandem cells examined in this study. This is ascribed to the relatively high Jsc as well as extremely high Voc, in spite of the relatively low FF. The photovoltaic performance of DSSCs was measured with a black metal mask (0.16 cm2). [a] Electrolyte: 0.22 M Co(bpy)32 + , 0.05 M Co(bpy)33 + , 0.05 M LiClO4, and 0.8 M TBP in CH3CN. TiO2 thickness: 3.5 mm (active layer). [b] Electrolyte: 0.6 M DMPII, 0.1 M LiI, 0.05 M I2 and 0.5 M TBP in CH3CN. TiO2 thickness: 16 mm (active layer 12 mm and scattering layer 4 mm).

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Figure 5. (a) Device structure of a tandem DSSC. (b) J¢V characteristics of the tandem DSSC with JK303/HC-A1 (top) and N749/HC-A4 (bottom). (c) Corresponding IPCE action spectra.

4. Conclusions As one of the methods for improving the efficiency of DSSCs, we investigated series-connected tandem DSSCs using a Co(bpy)32 + /3 + redox couple. Dye-sensitized tandem solar cells were optimized to improve their current density (Jsc) and open-circuit voltage (Voc). In this system, the top cell is made up of a transparent cell and



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Full Paper Table 3. Photovoltaic parameters of the individual DSSCs and the corresponding tandem DSSC under one-sun illumination. Device

Jsc (mA cm¢2)

Voc (mV)

FF (%)

PCE (%)

Top cell (JK303/HC-A1)[a] Bottom cell (N749/HC-A4)[b] Tandem cell

10.4 8.2 10.3

964 730 1,664

63.5 77.2 63.1

6.3 4.64 10.8

the bottom cell utilizes only the light passing through the top cell. We investigated several combinations of various dyes and coadsorbents in tandem-type DSSCs. Among them, the best efficiency was 10.8 % (Jsc = 10.3 mA cm¢2, Voc = 1.664 V, and FF = 63.1 %) for a series-connected tandem DSSC comprising JK303/HC-A1 (top cell) with the Co(bpy)32 + /3 + redox couple and N749/HC-A4 (bottom cell) with the I¢/I3¢ redox couple. To the best of our knowledge, the extraordinarily high Voc (> 1.66 V) is the highest value for dye-sensitized tandem solar cells reported to date.

Acknowledgements This work was supported by the Human Resources Development Program (No. 20124010203190) and the International Collaborative Energy Technology R&D Program (No. 20148520011250) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea, and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2014R1A2A1A10051630).

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Received: December 23, 2014 Accepted: March 29, 2015 Published online: May 13, 2015


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